Evauation of the Applicability of SWAT Mofel for Water shed Modeling: upper of wabi Shebelle River Basin, Ethiopia

Abstract

Field evaluations of irrigation systems play a fundamental role in improving surface irrigation design and management. This study evaluated performance of large scale irrigation scheme, Metahara sugar estate, in Awash River basin, Ethiopia. Field evaluations of on-farm irrigation systems under operation were extensively performed, aiming at improving both systems operation and irrigation scheduling. Field measurements were conducted during selected irrigation events. Soil, crop, and climatic data have been collected and analyzed. Irrigation scheduling and field application performances have been evaluated followed by optimization to improve irrigation scheduling and system operation. Microsoft excel optimizer 'solver', CROP WAT 8 , and SIRMOD III model were used to optimize infiltration parameters, calculate potential evapotranspiration and optimize field application performance respectively. The result showed that most of the irrigation events (about 80%) were late in timing that means they were made after management allowable deficit was reached. The design net depth of application of 75mm is lower than readily available water holding capacities of major soils in the root zone. Average gross applied depth of irrigation were 136mm in lOOm furrows and 207mm in 200m furrows which are by far bigger than the design gross application of 103mm. The field application performance evaluated against revised target depth of application showed that application efficiency, requirement efficiencies, and distribution uniformities were generally higher in lOOm furrows and lower in 200m furrows. It was also found that the performances were affected by irrigation timing. To improve the irrigation scheduling different net depth of application for major soils are recommended. These are llOmm in heavy clays and clays, 104mm in clay over loamy soils and 87mm in loam soils. Irrigation interval varying across soil type, season, and crop stages were determined. The result showed that determined irrigation intervals generally are shorter than the existing "theoretical irrigation intervals" .To improve operation management cut off time determined using SIRMOD III simulation for selected inflow rates are recommended.